You can use the relative time format next Sunday 17:00
. Like this:
$now = new DateTime();
$future_date = new DateTime('next Sunday 17:00');
$interval = $future_date->diff($now);
echo $interval->format("%d days, %h hours, %i minutes, %s seconds");
Output:
6 days, 2 hours, 33 minutes, 53 seconds
Read more about relative time formats here: http://www.php.net/manual/en/datetime.formats.relative.php